Windows can reflect a surprising amount of solar radiation. In some cases, this reflected radiation can become problematic. A certain amount of energy is, of course, carried in the solar radiation reflected off the exterior of a window. When this radiation falls on a nearby surface, the surface can be discolored. While this can occur even with a window having clear uncoated glass, the problem can be more significant when the window bears a coating that is highly reflective of solar radiation. This problem can also be more significant if the panes of the window in question have become inwardly cupped. (The panes of an IG unit can become cupped, for example, during cold weather when gas in the interior of the unit contracts.) The concave exterior pane of such a window would concentrate its reflected radiation at a focal point exterior to the window. This focal point would tend to move as the sun moves across the sky, thus potentially leaving elongated paths of discoloration.
As noted above, solar reflection problems can be particularly significant for windows and other glazings (e.g., doors, skylights, etc.) that bear reflective coatings, such as low-emissivity coatings. Low-emissivity coatings are well known in the present art. These coatings commonly include one or more reflective silver layers and two or more transparent dielectric layers. The silver layers in these coatings are highly reflective of infrared radiation. Thus, they favorably reduce the transmission of radiant heat through the coating. However, these coatings also tend to have relatively high solar reflectance. For example, a window bearing a conventional low-emissivity coating would typically have a solar reflectance of at least about 30%–35%, while the solar reflectance of a window having clear uncoated glass would typically be around 13%. Thus, from the perspective of solar reflection problems, conventional low-emissivity coatings are less than ideal. Accordingly, it would be desirable to provide a low-emissivity coating that has low solar reflectance.
It would be particularly desirable to provide a low-emissivity coating that has low solar reflectance and also provides significant shading properties. As is well known, the solar heat gain coefficient (SHGC) of a window is the fraction of incident solar radiation that is admitted through a window. There are a number of applications where low solar heat gain windows are of particular benefit. In warm climates, it is especially desirable to have low solar heat gain windows. For example, solar heat gain coefficients of about 0.4 and below are generally recommended for buildings in the southern United States. Similarly, any windows that are exposed to a lot of undesirable sun preferably have a low solar heat gain coefficient. For example, windows on the east or west side of a building tend to get a lot of sun in the morning and afternoon. Likewise, sunrooms, solariums, and greenhouses typically get a great deal of sun. For applications like these, the solar heat gain coefficient plays a vital role in maintaining a comfortable environment within the building in question. Thus, it is beneficial to provide windows of this nature with coatings that establish a low solar heat gain coefficient (i.e., high shading ability coatings).
A tradeoff is sometimes made in high shading ability coatings whereby the films selected to achieve a low SHGC have the effect of restricting the visible reflectance to a higher level than is desired. As a consequence, windows bearing these coatings may have a somewhat mirror-like appearance. It would be desirable to provide a high shading ability coating that has sufficiently low visible reflectance to obviate this mirror-like appearance problem.
In addition to having undesirably high visible reflectance, the transmitted and reflected colors of conventional high shading ability coatings tend not to be ideal. For example, these coatings commonly exhibit hues that are more red and/or yellow than is desired. To the extent a coating has a colored appearance, it is pleasing if the coating exhibits a transmitted and/or reflected hue that is blue or blue-green. The chroma of these coatings tends also to be greater than is desired. In most cases, it is preferable to provide a coating that is as color neutral (i.e., colorless) as possible. Thus, the reflected and transmitted colors of conventional low solar heat gain coatings tend to be less than ideal, both in terms of hue and chroma.
U.S. Patent Application No. 60/376,826 (Hoffman), the entire contents of which are incorporated herein by reference, discloses advantageous low-emissivity coatings that have low solar reflectance. These coatings achieve an exceptional combination of properties, including particularly low solar reflectance. In the '826 application, Hoffman describes five uniquely preferred low solar reflectance, low-emissivity film stacks. These film stacks are exceptionally well suited for a variety of applications. However, it would be desirable to improve these film stacks in such a way that they impart greater insulating ability in windows. For example, it would be desirable to achieve substantial decreases in emissivity and U Value. Unfortunately, the changes required to decrease emissivity and U Value would be expected to cause an attendant decrease in visible transmittance and/or an attendant worsening of reflected or transmitted color. As skilled artisans will appreciate, overcoming this problem is an exceedingly difficult task, particularly considering the presence of the high absorption primary layer in these coatings, which renders coating design highly unpredictable.